Tensile strength of reservoir rock is one of the most important factors in designing hydraulic fracturing for reservoirs comprising sedimentary rocks. This includes tight shale gas reservoirs, tight sedimentary sandstone and chalk reservoirs. Tensile strength is also important in the determination of wellbore stability of wells drilled in sedimentary rocks. Tensile strength is one of the determinant parameters for the pumping rate of aqueous fluids in hydraulic fracturing and for using an appropriate mud weight in drilling. Mechanical tests on natural rock samples involve large instruments and complex procedures, in part, because significant force is required to break the rock. In addition, the laboratory mechanical tests require special sample preparation and the results are often unreliable, preventing measurement of tensile strength at the wellsite.
Traditional mechanical force testing methods use external force to measure tensile strength and include Brazilian Test methods and pull methods. The Brazilian Test method involves an indirect measurement of the tensile strength by compressing the unconfined rock. Pull tests require specific sample shapes and misalignment in setting up the sample produces incorrect results. In the Brazilian Test, there is a risk of producing tensile failure at the ends by subjecting the specimens to compressive force along their length until the flat ends split.
Therefore, a method that provides direct reliable measurement of material tensile strength with less equipment is desired.